Raitis Veinbahs more about me my toy projects my github my asciinema

Python cannot even `uniq'

December 16, 2015

Almost everywhere but…

It’s ridiculous — ruby, haskell, even sh has it!

λ: import Data.List # this is haskell
λ: nub [1,2,3,4,5,6,1,2,3,6,4,3,5,3,2,4,5,3,1,3,5]

irb(main):001:0> (b = [4, 'a', 'b', 'c', 9, ['a'], 1]).uniq == b
=> True # this is ruby

Python doesn’t have built-in code for removing duplicates from a list (without messing up the order of the list).


I thought that it’s any standard library’s sacred duty to provide useful utilities to us, the programmers. Speed and code reuse should be our goal and could be achieved by avoiding having tiny of oneliners all over the place! (Which is a real problem given that `uniq’ is a common function.)

Acknowledging the fact that python hasn’t fulfilled its duty, I present some of my own code.


Ok, here’s what I have:

b = [4, 'a', 'b', 'c', 9, ['a'], 1]
c = [4, 'a', 'b', 'c', 9, 1]
uniq1 = lambda a: (lambda b: [b.append(x) or x for x in a if x not in b])([])
uniq2 = lambda a: list(set(a))

print(uniq1(b) == b)      # line1
print(uniq1(c) == c)      # line2
print(uniq2(c) == c)      # line3
try: print(uniq2(b) == b) # line4
except TypeError: print('because lists are unhashable, set(b) cannot exist')

because lists are unhashable, set(b) cannot exist

# BONUS! Uniq by key:
from itertools import groupby
groupsort = lambda l, key: [(k, list(g)) for k, g in groupby(sorted(l, key=key), key)
uniqkey   = lambda by, a: [list(b)[0] for _, b in groupsort(a, by)]

Note: simplest example for getting a TypeError of this kind is set([[]]).


Supposedly, we can just use list(set(a)) and I’m just whining about nothing. And knowing list(set([1,2,3,1])) == [1,2,3] you’d guess that line3 = True! Well you just got tricked and were lucky that hash of a numbers is the number itself, hash(1234) => 1234. Their order won’t therefore remain the same in general. hash('foo') => -740391237 So 3. is false.

The orderless version uniq2 can’t even handle b. Due to reasons already mentioned in the note above.

Another outrageous argument that I’d like to refute is that because the order is ambiguous we shouldn’t write this function at all since which one one should we keep in [1,2,1]. Why not just decide on some default order that would still be totally useful — first copy stays.

This should’ve been in the standard library. Weak, python, weak.